The present invention generally relates to oxide bronze compositions. The invention more particularly relates to methods of making oxide bronze compositions and articles formed in accordance therewith.
Oxide bronzes, as described in A. F. Wells, Structural Inorganic Chemistry, 5th ed., pp. 612-625 (copyright Oxford University Press 1974, 1984), incorporated herein by reference, typically include solid oxide phases of transition metals such as tungsten, molybdenum, niobium, rhenium, titanium, vanadium, or the like that are doped with cations from the alkali, alkaline earth or lanthanide group elements. The oxide bronzes are characterized by intense color, metallic conductivity or semiconductivity and resistance to attack by non-oxidizing acids. As discussed herein, the terminology xe2x80x9coxide bronzexe2x80x9d is used in accordance with the present invention to refer to this subgroup of solid oxide phases, distinguishing them from other transition metal oxides and from the purely metallic copper-based alloys called simply xe2x80x9cbronze.xe2x80x9d
Oxide bronzes have been extensively studied, and properties of interest for applications have been found. For example, superconductivity has been found in certain compositions at very low temperatures (i.e. helium boiling point range, approximately 4.2 K). More recently, an observation has been made as to the possibility of superconductivity in sodium tungsten bronze phase formed near the surface of a WO3 crystal (xe2x80x9cNa0.05WO3,xe2x80x9d High-Tc Update, Vol. 13, No. 9, p.1 May 1, 1999; Reich et al., xe2x80x9cPossible Nucleation of a 2D Superconducting Phase on WO3 Single Crystals Surface Doped With Na+xe2x80x9d The European Physical Journal B, vol. 9, 1-4; both of which are incorporated herein by reference). This suggestion specifically relates to the observation of possible superconducting characteristics and a superconducting transition temperature (Tc) of approximately 91 K in Na-doped WO3 crystals having a surface composition of Na0.05 WO3. The possible superconductivity is suggested to occur near the surface of the sodium-doped crystal. Other properties of the oxide bronzes of interest for applications include dielectric, piezoelectric and electro-chromic properties.
For applications requiring large volumes or areas of material, single crystals are usually not practical, and some kind of polycrystalline structure is required. In addition, oxide bronzes in which the grains are not textured or aligned to reduce or eliminate weak link couplings between grains can pose significant limitations on the fabrication and practical use of certain devices formed of such materials. For example, for long-length polycrystalline superconducting wires, it can be essential to avoid weak link couplings between grains which limit supercurrent density.
In the class of high temperature superconductor (HTS) cuprates (all of which are not in the class of oxide bronzes), various techniques for polycrystalline texturing are utilized. It is generally sufficient, for example, in the case of the BSCCO-based HTS materials to align the grains such that only the c-axis is aligned in the same direction (uniaxial alignment). It is generally not necessary that the a-b planes be aligned. The required c-axis texture in the BSCCO materials can be obtained utilizing various deformation processes, e.g. a thermal-mechanical deformation process.
In the YBCO HTS systems, it is typically necessary to align the grains in the a-, b-, and c-axis directions (biaxial alignment), but the [100] direction must be aligned in the current direction. This can be accomplished for example by growing the YBCO grains on a textured template structure that has a unique crystallographic orientation called xe2x80x9ccube texturexe2x80x9d. The texture requirement in this case has been shown to lie within 10 and preferably 5 degrees of misorientation angle between grains, if no significant diminution of current carrying capacity is to occur. The oxide bronzes may also need to be biaxially aligned for the highest current carrying capacity. In contrast to the YBCO systems, however, the oxide bronzes can have many other textures besides cube texture.
In the context of high temperature superconductor (HTS) applications, it would therefore be desirable to provide polycrystalline structures that are advantageous over the prior art. In particular, fine-grained materials would be desirable for excellent homogeneity if grain boundary weak links are not an obstacle. It would also be desirable to provide oxide bronze superconducting compositions having grains sufficiently textured or aligned as to overcome weak link characteristics of the grain boundaries, thereby allowing transport of high current across such grain boundaries. Other properties of oxide bronze compositions may also be optimized by providing textured polycrystalline or granular structures. For example, the dielectric breakdown field of oxide bronze capacitors may be increased by texturing.
The present invention provides methods for making oxide bronze compositions and articles incorporating such compositions. More specifically, the present invention describes procedures for the synthesis of oxide bronzes into forms that can be useful for the fabrication of practical devices.
As used herein, xe2x80x9coxide bronzexe2x80x9d in accordance with the present invention include materials having the general formula AxBOy, in which A is an alkali metal (e.g., Li, Na, K, Rb or Cs), an alkaline-earth metal (e.g. Mg, Ca, Sr or Ba), a lanthanide metal (e.g. La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu) or one of: In, Cu, Sn, Pb, Tl or Ag and in which A has a valence, m, of 1, 2 or 3; B is a transition metal (e.g. Ti, V, Nb, Ta, Mo, W, Re or Ru) and has a valence, n, of less than or equal to 6; 0 less than x less than 1; and y=[(x)(m)+n]/2. In some embodiments, 0.001 less than x less than 0.3 and 2.8 less than y less than 3.05.
In some embodiments, the oxide bronzes are based on a host structure having a composition of BO2 or BO3, with the valence, n, of B being respectively less than or equal to 4 or 6. The cation Am (where m is the valence of A and is typically 1, 2 or 3) can be incorporated into the BO2 or BO3 structure if a portion, (x)(m), of the Bn atoms in BOy are reduced to Bnxe2x88x921. The electrical properties of the oxide bronzes are associated with the fact that no distinction can be made between the Bn and Bnxe2x88x921 atoms in the lattice. The extra electrons, (x)(m), per mole are delocalized over the lattice.
The present invention thus relates to unique oxide bronze compositions and structures and methods of making such structures. Mere preparation of oxide-based bronzes (e.g., NaxWO3) in bulk form, however, is not sufficient to render them useful for the applications described herein. In order to produce useful articles and devices, the present invention therefore provides, in one aspect, polycrystalline oxide bronze structures having very small, fine grains (e.g., typically in the micron or submicron range in their maximum dimension), which can be for example, rod-like, or equi-axed. Small grains provide the polycrystalline material with mechanical robustness, and improved sintering and density characteristics. In the case of oxide superconductors in which the current capacity within each grain (crystal) varies systematically within the grain relative to position or direction, fine grains tend to reduce the maximum length scale of inhomogeneity, providing for a macroscopically more homogeneous superconductor with improved properties. Grains of larger dimensions (i.e., a few hundred xcexcm) can also be satisfactory for some utilities of the present invention. In coated conductors, single crystal may be the most suitable form, and small grain sizes may not be favorable. Therefore, in some embodiments, the polycrystalline superconducting layer is a thin continuous sheet disposed on the substrate, in which the average grain size is epitaxially related to the grain size of the underlying substrate. In other embodiments, the polycrystalline superconducting layer is a filament embedded in a matrix and comprises small, fine grains in at least one direction. In general, there is no restriction on grain size; both large ( greater than 10 micron) and small ( less than 10 micron) grain sizes can be suitable for varying applications. The grain size can be primarily dictated by the grain size of the substrate and when based on roll-textured metals, is typically greater than 10 microns. The epitaxially grown oxide bronze superconductor generally adopts the grain size of the substrate.
Another aspect of polycrystalline oxide bronze structures addressed by the present invention includes structures in which the grains are strongly textured. Texture can be characterized by an x-ray rocking curve or pole figure in which the full-width-half-maximum (FWHM) preferably is less than or equal to 20 degrees wide (and more preferably less than or equal to 10 degrees). Highly textured structures can provide a variety of advantages. In long-length polycrystalline superconducting wires for example, it can be essential to avoid weak-link couplings between grains which limit the current density. In addition to allowing transport of high current across grain boundaries, other properties of oxide bronze compositions may be optimized by providing textured polycrystalline or granular structures. For example, the dielectric breakdown field of oxide bronze capacitors may be increased by texturing.
Structures in which weak-link coupling is avoided and in which fine grain sizes are provided can be achieved simultaneously in certain cases to provide articles and devices having even more favorable properties.
In preferred embodiments, the compositions and textured articles formed utilizing such compositions are suitable for use as superconducting devices. For example and while not to be construed as limiting, oxide bronze textured articles formed in accordance with the present invention include superconducting products such as power transmission cables, motors, generators, transformers, fault current limiters, magnets, electronic applications and the like. Exemplary applications in the electronics area include, bit are not limited to, passive communications applications such as antennae, wave guides, filters and interconnects, and data processing or analog applications based on active devices such as Josephson junctions and superconducting field effect transistors. It is anticipated that when these articles are formed in accordance with the invention, high temperature superconducting devices that exhibit high superconducting transition temperature Tc (e.g., greater than 50 K) and high critical current densities will be provided. In preferred embodiments, the Tc will be greater than 77 K and in particularly preferred embodiments, the Tc will be significantly greater than 77 K (e.g. 90 K and above). Critical current densities are expected to be greater than 100 kA/cm2 at Tc/2. These materials may have a perovskite-like structure, often with a cubic, hexagonal or tetragonal unit cell. Even in cases of lower symmetry, however, it is expected that the inherent electrical and magnetic properties of the grains in these materials will be significantly more isotropic than the high temperature superconducting materials based on the cuprates. In cases where the texturing methods of this invention are utilized, the inherent bulk properties are expected to be much more isotropic relative to HTS cuprates and to provide desirable superconducting properties. An aspect of this invention is the recognition that the grains can be relatively equi-axed in shape, affording more nearest neighbor contacting grains than in the anisotropic superconductors. This, in combination with isotropic current transport, allows for higher current capacity in a bulk article with a lower degree of texturing than in cuprate-based HTS oxides. In one embodiment of the present invention, methods for fabricating the oxide bronze superconductors into textured structures which are useful in practical applications (particularly in the form of long-length flexible wires) are provided.
In addition, it is anticipated that other properties (e.g. dielectric, piezoelectric and electrochromic properties) of the oxide bronze compositions and articles of the present invention may be useful for fabrication of practical non-superconducting devices. For example, optical applications in which a textured article may provide an advantage include electrochromic devices. In addition, ferroelectric devices such as capacitors and piezoelectrics may be desirable to texture in accordance with the present invention. In one aspect of the invention, textured articles including the non-stoichiometric compound NaxWOy and exhibiting various magnetic, electrical, and optical characteristics are provided. In these embodiments, it x may be 0.001 less than x less than 0.3.
The foregoing has outlined some of the more pertinent aspects of the present invention. These aspects should be construed to be merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention as will be described. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the following Detailed Description of the Preferred Embodiments.